As an example of how to separate a supernatant from a precipitate by vacuum filtration, the movie explains the separation of 4-methoxy-2-nitroacetanilide yielded in the following reaction which is carried out in "Organic Chemistry Experiment".
This filtration technique uses a water-jet pump or an aspirator. The reduced pressure makes the filtration fast.
This is suitable for separation of a precipitate consisting of light and fine particles.
Organic precipitates in synthetic experiments are usually collected by vacuum filtration.
Because a flat filter paper is employed, the collected precipitate is easily raked.
Understanding how to use a three-way stopcock is desirable. → Three-Way Stopcock
How to Operate Vacuum Filtration
Preparation
Connect a filtration chamber and an aspirator with thick wall rubber tubing through a three-way stopcock.
Attach a rubber adapter to a Buchner funnel and set the funnel at the top of the filtration chamber.
Put a receiving container in the filtration chamber. If the container is too far from the tip of the funnel, place a rubber board underneath.
Wet the contact between the bottom rim of the filtration chamber and the coaster glass plate. Slightly rubbing their ground glass faces to obtain a good contact.
Put a filter paper slightly smaller than the inside of the funnel and add a small amount of a solvent to wet the filter paper.
Turn on the aspirator and open the three-way stopcock to fit the filter paper to the funnel.
Filtration and Washing of the Precipitate
Pour the sample into the funnel with vacuuming.
The remaining precipitate in the container is transferred into the funnel as follows. Release the vacuum in the filtration chamber via the three-way stopcock. Transfer the filtrate back to the original container. Pour the mixture onto the filter paper again via vacuum filtration.
After transferring all the precipitate onto the filter paper, wash the precipitate as follows. Release the vacuum in the filtration chamber via the three-way stopcock. Pour a suitable solvent to the funnel to soak the precipitate on the filter paper. Suck the liquid by applying vacuum via the three-way stopcock. Repeat the operations a few times.
Suck the liquid thoroughly under vacuum by gently pressing the precipitate on the filter paper with a bottom of a small beaker.
After the experiment, release the vacuum in the filtration chamber via the three-way stopcock, and then off turn the aspirator. If the aspirator is turned off without releasing the vacuum, water backflushes into the filtration chamber!